Method release

unittests/catch.hpp:9513–9516

        }

        void release( std::size_t index ) {
            m_streams[index]->copyfmt( m_referenceStream ); // Restore initial flags and other state
            m_unused.push_back(index);
        }

        // !TBD: put in TLS
        static auto instance() -> StringStreams& {